Stroke occurs in approximately 600,000 people in the United States yearly and is associated with significant morbidity and mortality. In 1998, 158,448 people died as a result of stroke. Over 45 billion dollars of health care costs are attributable to stroke in a year. Approximately 4.5 million stroke survivors are alive today. 2001 Heart and Stroke Statistical Update, American Heart Association.
There are a number of important causes of stroke. These include thrombotic, embolic, and hemorrhagic causes. Embolic strokes account for approximately one quarter of all strokes. In embolic strokes, a blood clot or other material travels to the brain from another site and occludes a blood vessel, thereby depriving the brain of the needed blood flow (and associated oxygen and glucose supplies); this results in the death of the cells that are usually supplied by this blood flow. The most common source for these emboli is the heart. Blood clots, otherwise known as thrombi, can form in patients with atrial fibrillation, heart valves, artificial materials (e.g., artificial hearts, vascular stents, and the like), or significant heart disease. It is estimated that 15 percent of all strokes occur in patients with atrial fibrillation. 2001 Heart and Stroke Statistical Update, American Heart Association.
The normal electrical rhythm of the top chambers of the heart (i.e., the atria) is sinus rhythm. Normal sinus rhythm is typically characterized by a rate of about 60 to about 100 beats/minute under normal circumstances. With exercise, the heart rate may increase up to about 180 beats/minute. In normal sinus rhythm, the electrical activation of the atrium results in contraction of this chamber and emptying of blood into the ventricles (the lower chambers of the heart). Atrial flutter or fibrillation are abnormally rapid and/or chaotic rhythms that are characterized by rates of about 300 to 400, or more, beats/minute in the atrium. Instead of a single wavefront of electrical activation with sinus rhythm, atrial fibrillation presents multiple, simultaneous wavefronts. Because of the rapid rate and the multiple electrical wavefronts, there is no significant coordinated atrial contraction. Because of the lack of atrial contraction in patients with atrial fibrillation, the blood flow through the atrium may be sluggish, allowing for the formation of blood clots. If these blood clots leave the atrium and travel to the brain, they may cause an embolic stroke. Because of the high risk for thromboembolism (i.e., formation of blood clots and their travel through the vascular system) in many patients with atrial fibrillation, these patients are typically treated with a blood thinner (e.g., coumadin) to help prevent blood clot formation. The use of blood thinners such as coumadin can reduce the risk of stroke by up to about 75 percent. Unfortunately, some patients cannot take coumadin due to the risk of bleeding; they may, for example, be at a high risk for complications or have bleeding problems. It is estimated that up to about 5 percent of patients develop major bleeding as a result of coumadin therapy.
There are other medical conditions also associated with thrombus formation that are currently treated with coumadin. Patients with artificial valves or other implanted artificial materials are frequently treated with coumadin to prevent thrombus formation. Blood clot formation on an artificial valve or other implanted surface can cause stroke and/or can prevent the artificial valve or other implant from functioning properly. Patients may also be at risk for forming blood clots in their blood vessels or chambers due to poor flow characteristics. These conditions are treated with coumadin.
Currently, there are no effective alternatives to the use of coumadin or other blood thinners for the treatment of these patients. It would be desirable, therefore, to provide alternative methods for preventing or significantly reducing the risk of thrombus formation. It would also be desirable to provide methods for preventing or significantly reducing the risk of thrombus formation which could be used separately or in combination with blood thinners to provide treatment options for essentially all patients at risk for thrombus formation. The present invention provides such methods as well as devices for carrying out such treatment methods.